Non-quenched and tempered steel having ultrafine grained pearlite structure and method of manufacturing the same

ABSTRACT

This invention relates to a method of manufacturing non-quenched and tempered steel having an ultrafine grained pearlite structure, including hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body; rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body; isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range so as to be isothermally transformed; and air-cooling the hot forged body; and to non-quenched and tempered steel manufactured thereby.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No. 10-2011-0053074, filed on Jun. 2, 2011, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-quenched and temperature (non-QT) steel having an ultrafine grained pearlite structure by inducing pearlite low-temperature isothermal transformation while suppressing the formation of proeutectoid ferrite and pearlite during continuous cooling after hot forging, and to a method of manufacturing the same.

2. Description of the Related Art

Automobile parts are being manufactured via hot forging using medium-carbon steel. Typical forged part manufacturing processes include hot forging medium-carbon steel (or alloy steel) as a forging material in an austenite range, followed by quenching and tempering. The hot forged steel is transformed into martensite during quenching, which is then decomposed into ferrite and carbide during tempering, thus obtaining forged steel (which is referred to as quenched and tempered (QT) steel) having a martensite structure with high strength and high toughness. However, the typical manufacturing of hot forged steel is problematic because quenching and tempering must be additionally performed after hot forging, which undesirably complicates the post-heat treatment process after forging and increases the manufacturing cost.

With the goal of solving such problems, microalloying (MA) forged steel (which is referred to as non-QT steel) has been developed. MA-forged steel has mechanical properties similar to those of QT steel, and is formed by performing only a controlled cooling process (without carrying out the QT processes) after a forging process. Currently, MA-forged steel is utilized in lieu of typical QT-forged steel in the fabrication of many hot forged parts for automobiles. MA-forged steel, which is designed from the physical metallurgical point of view, is composed mainly of medium-carbon steel, and additionally includes small amounts of microalloying elements such as V, Nb, Ti, etc., thus depositing a carbonitride. As such, the formation of coarse austenite crystal grains during hot forging is suppressed, thus forming a fine pearlite-ferrite structure. Further, the deposition of fine carbonitride is induced in ferrite upon austenite-ferrite transformation, thus strengthening pearlite-ferrite, and enhancing the strength and toughness of forged steel.

The non-QT MA-forged steel is advantageous because it is manufactured using only controlled cooling without complicated post-heat treatment, thus ensuring a low manufacturing cost while still providing strength similar to QT steel and superior fatigue properties. However, the non-QT MA-forged steel is disadvantageous because its toughness is inferior to QT-steel. In order to solve this problem, thorough research in the development of hot forged steel has been conducted worldwide for the past 20 years, and furthermore continues to be conducted.

The non-QT MA-forged steel is composed mainly of medium-carbon steel, and has a basic structure of pearlite-ferrite. The mechanical properties of the pearlite-ferrite structure are determined by the pearlite fraction, the size of colonies, and the lamellar spacings. In a conventional hot forging process, in order to control the fine pearlite-ferrite structure, the cooling rate is controlled in the controlled cooling process after hot forging.

In the controlled cooling process, when the cooling rate is slow, the proeutectoid ferrite fraction is increased and the pearlite fraction is decreased. Ultimately, the strength is reduced but toughness is enhanced. In contrast, when the cooling rate is fast, the proeutectoid ferrite fraction is decreased and the pearlite fraction is increased, and thus the strength is increased but toughness is reduced. In the latter case, strength is increased but toughness is decreased because (1) the ferrite fraction is decreased, and (2) the formed pearlite structure, namely, the pearlite colonies and the lamellar structure, are coarse. The reason the pearlite structure is coarse is because the pearlite is formed during continuous cooling. In particular, while proeutectoid ferrite is primarily formed during continuous cooling, the carbon concentration accumulates and is increased at the interface of austenite/ferrite, and thus pearlite is easily formed at high temperature. The pearlite formed at high temperature has a small degree of supercooling and, thus, a low rate of nucleation. Thus, a small number of nuclei are formed and the formed high-temperature pearlite easily becomes coarse as continuous cooling is carried out. As a consequence, the coarse pearlite colony structure is formed. While the strength may be maintained at the basic strength level of pearlite, the toughness of pearlite is not sufficiently ensured and the ferrite fraction is also low, thereby drastically reducing the toughness of forged steel.

Accordingly, there are limitations to enhancing strength and toughness of conventional pearlite-ferrite steel using controlled cooling. In attempt to solve such problems, conventional techniques have focused mainly by using new alloy designs. However, it is expensive to add elements in order to prepare new alloys, and despite the addition of such alloying elements, the improvement in performance has not been significant. For example, the conventional hot forging process of non-QT steel has been modified by controlling the cooling process after forging in order to optimize the ferrite fraction and the pearlite colony structure, with the intended result of increasing strength and toughness. However, in such pearlite-ferrite forged steel, when the cooling rate is decreased, the ferrite fraction is increased and the pearlite fraction is decreased. As a result, toughness is enhanced but strength is reduced. In contrast, when the cooling rate is increased, the ferrite fraction is decreased and the pearlite fraction is increased, and as a result strength is enhanced but toughness is reduced. Thus, in the conventional pearlite-ferrite forged steel, the production of forged steel having superior strength and toughness by controlling only the cooling rate is limited.

This related art is merely utilized to enhance understanding about the background of the present invention, and will not be regarded as conventional techniques known to those having ordinary knowledge in the art.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems encountered in the related art, and an object of the present invention is to provide a new concept of high-strength and high-toughness non-QT steel, and in particular one that comprises an ultrafine grained pearlite (pseudo-pearlite) structure which does not include ferrite. It is further an object of the present invention to provide such a material by applying a novel controlled cooling process, and wherein it is not necessary to change the alloy composition.

An aspect of the present invention provides a method of manufacturing non-QT steel having an ultrafine grained pearlite structure, composed mainly of Fe and additionally of about 0.43˜0.47 wt % of C, about 0.15˜0.35 wt % of Si, about 1.1˜1.3 wt % of Mn, about 0.03 wt % or less of P, about 0.04 wt % or less of S, about 0.3 wt % or less of Cu, about 0.2 wt % or less of Ni, about 0.1˜0.2 wt % of Cr, about 0.05 wt % or less of Mo, about 0.08˜0.15 wt % of V, about 0.02 wt % or less of Al, and other impurities, wherein “or less” refers to any amount below the stated value and greater than 0 wt %. In accordance with this aspect, the method comprises hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body; rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body; isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range, thus isothermally transforming it; and air-cooling the hot forged body.

In this aspect, hot forging may be performed at a suitable temperature that results in hot-temperature compression deformation of the material, for example about 1000˜1250° C. Rapid cooling may be performed by cooling the hot forged body to a suitable low-temperature pearlite transformation range, for example about 500˜600° C., at a suitable cooling rate, for example about 10° C./s or more. Isothermal transformation may be performed by isothermally holding the supercooled hot forged body at the low-temperature pearlite transformation range, for example about 500˜600° C., for a suitable time, for example about 5˜30 minutes.

In an exemplary embodiment, the hot forged body is rapidly cooled to 500˜600° C. at a rate of 10° C./s or more, and the supercooled hot forged body is then isothermally transforming by isothermally holding it at 500˜600° C. for 5˜30 minutes.

Another aspect of the present invention provides non-QT steel having an ultrafine grained pearlite structure, produced by subjecting a hot forged steel material at about 1000˜1250° C. to rapid cooling to a pearlite transformation range of about 500˜600° C., then isothermal holding at the corresponding temperature for 5˜30 minutes, and then air-cooling.

In this aspect, the hot forged steel material may be rapidly cooled to the pearlite transformation range of about 500˜600° C. at a rate of about 10° C./s or more.

In this aspect, the non-QT steel formed after air-cooling may have a proeutectoid ferrite fraction of about 5 vol % or less and pearlite colonies having a size of about 5˜10 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show the microstructures of non-QT steel which is hot forged at 1150° C., and FIGS. 1C and 1D show the microstructures of non-QT steel which is hot forged at 1050° C.;

FIG. 2A shows the microstructure obtained by cooling the hot forged steel to 600° C. at 2° C./s and then air-cooling it, and FIG. 2B shows the microstructure obtained by cooling the hot forged steel to 600° C. at 10° C./s and then air-cooling it;

FIG. 3A shows the microstructure obtained by cooling the hot forged steel to 600° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it, FIGS. 3B and 3C show the microstructures obtained by cooling the hot forged steel to 550° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it, and FIG. 3D show the microstructure obtained by cooling the hot forged steel to 500° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it;

FIG. 4 is a graph showing the effects of isothermal holding temperature on Vickers micro-hardness;

FIGS. 5A and 5B show the microstructures after respectively air-cooling and multi-stage rapid cooling of steel hot forged at 1100° C.; and

FIGS. 6A and 6B show other comparison results of FIGS. 5A and 5B.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, embodiments of the present invention regarding non-QT steel having an ultrafine grained pearlite structure and a method of manufacturing the same will be described in detail while referring to the accompanying drawings.

According to the present invention, the method of manufacturing non-QT steel having an ultrafine grained pearlite structure includes hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body; rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body; isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range so as to be isothermally transformed; and air-cooling the hot forged body.

The steel material is composed mainly of Fe, and can contain additional components such as one or more of C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Al and other impurities. For example, the steel material can contain one or more of these components in ranges of about 0.43˜0.47 wt % of C, about 0.15˜0.35 wt % of Si, about 1.1˜1.3 wt % of Mn, about 0.03 wt % or less of P, about 0.04 wt % or less of S, about 0.3 wt % or less of Cu, about 0.2 wt % or less of Ni , about 0.1˜0.2 wt % of Cr, about 0.05 wt % or less of Mo, about 0.08˜0.15 wt % of V, about 0.02 wt % or less of Al, and other common impurities, which are shown in Table 1 below. It is noted that the term “or less” when referring to these additional materials, can include any value greater than 0 wt % when that component is present.

TABLE 1 C Si Mn P S Cu Ni Cr Mo V Al 0.43~0.47 0.15~0.35 1.10~1.30 ~0.03 ~0.04 ~0.3 ~0.2 0.10~0.20 ~0.05 0.08~0.15 ~0.02

In various embodiments, the hot forging process is carried out at about 1000˜1250° C., and the rapid cooling process is performed so that the hot forged body is cooled to about 500˜600° C. at a rate of about 10° C./s or above. Also, the isothermal transformation process is performed so that the supercooled hot forged body is isothermally held at about 500˜600° C. for about 5˜30 minutes. The rapid cooling process may be conducted so that the hot forged body is cooled to about 500˜600° C. at a rate of about 10° C./s or more, and the isothermal transformation process may be conducted so that the supercooled hot forged body is then isothermally held at about 500˜600° C. for about 5˜30 minutes.

The non-QT steel having an ultrafine grained pearlite structure produced according to the above method may be characterized in that the steel material hot forged (e.g. at about 1000˜1250° C.) is rapidly cooled to the pearlite transformation range (e.g. about 500˜600° C.), isothermally held at the corresponding temperature (i.e. the pearlite transformation range) for about 5˜30 minutes, and then air-cooled.

In accordance with the present invention, the non-QT steel produced by subjecting the hot forged steel material to rapid cooling to the pearlite transformation range (e.g. about 500˜600° C. at a rate of 10° C./s or more) and then air-cooling can provide a proeutectoid ferrite fraction suppressed to about 5 vol % or less, and pearlite colonies having a size limited to about 5˜10 μm.

The present invention pertains to a multi-stage rapid cooling process for manufacturing non-QT V-MA medium-carbon forged steel which provides a new type of structure quite different from a conventional pearlite-ferrite structure. In particular, the present invention provides an ultrafine grained pearlite (pseudo-pearlite) structure, which is formed by a first stage of rapidly cooling the hot forged body to a low-temperature pearlite isothermal transformation range at a fast cooling rate (e.g. about 10° C./s or more) just after hot forging, a second stage of isothermally holding the supercooled hot forged body for a short period of time at the low pearlite isothermal transformation temperature to thus form an ultrafine grained pearlite colony structure, and a third stage of air-cooling the ultrafine grained pearlite (pseudo-pearlite) structure formed using isothermal transformation.

In accordance with the present invention, in the first stage the hot forged body is rapidly cooled to the low-temperature pearlite isothermal transformation range just after hot forging so that the transformation of proeutectoid ferrite formed during continuous cooling is maximally suppressed, and further so that the transformation of pearlite formed during continuous cooling is maximally suppressed.

When the phase transformation occurring during continuous cooling is suppressed in this way, an austenite hot forged body is obtained which is extremely supercooled up to the low-temperature pearlite isothermal transformation range.

In the second stage, the extremely supercooled austenite forged body is isothermally held for a short period of time at the low pearlite transformation temperature, whereby the rate of nucleation of pearlite colonies is greatly increased, thus obtaining an ultrafine grained pearlite colony structure.

The size of the pearlite colony structure is rendered smaller upon isothermal transformation than upon continuous cooling transformation. In the case of continuous cooling transformation, because the nucleation of pearlite occurs at a relatively high temperature, the rate of nucleation is low and the colonies also become coarse during cooling. On the other hand, in the case of the low-temperature isothermal transformation, the nucleation of colonies occurs at low temperature, and thus a driving force is very large, so that the rate of nucleation is greatly increased. Furthermore, using low-temperature isothermal transformation, the formation of coarse colonies is minimized, thereby obtaining the ultrafine grained colony structure. Upon air-cooling in the third stage, V-carbonitride is deposited on ferrite in pearlite, thus further strengthening the pearlite.

The ultrafine grained pearlite (pseudo-pearlite) structure thus formed has a very limited proeutectoid ferrite fraction (5% or less) and ultrafine grained pearlite colonies, thus remarkably enhancing its strength compared to that of conventional pearlite-ferrite forged steel. The structure further exhibits very good toughness to an extent comparable to conventional QT steel.

Below, the present invention is described in more detail with reference to the drawings.

First, in accordance with the present invention, a high-temperature deformation process, which is a hot forging process, were experimentally simulated using a high-temperature deformation simulator (Gleeble-1500), followed by a multi-stage rapid cooling process. Using Gleeble-1500, test steel (diameter 10 mm×height 15 mm) was homogenized at 1200° C. for 3 minutes, and hot deformed at 1150° C. As such, in order to simulate hot forging conditions, hot deformation was carried out under conditions of two-step compression at deformations of 0.4 and 0.8 at a predetermined deformation rate (5° C./s). The high-temperature compression deformed test sample was rapidly cooled to a low-temperature isothermal pearlite transformation range (500˜600° C.) at a fast rate of 10° C./s just after hot deformation. The rapidly cooled hot deformed body was subjected to isothermal pearlite transformation for a short period of time (30 minutes or less) in the temperature range of 500˜600° C., and then air-cooled.

Second, using multi-stage rapid cooling deduced from Gleeble testing, a simple hot forged product was manufactured and the tensile properties thereof were evaluated. To this end, a hot upsetting test (depression rate: 6 mm/s; deformation: 1.0˜1.2) was performed using a hot press, and then rapid cooling was carried out in the low-temperature isothermal pearlite transformation range (500˜600° C.). In accordance with embodiments of the present invention, in order to achieve rapid cooling and pearlite isothermal transformation, a Pb-bath isothermally held (500˜600° C.) was used. The test sample for the hot upsetting test had a diameter of 30 mm×a height of 40 mm. The cooling rate (based on the central portion of the test sample) obtained during rapid cooling after upsetting was 15° C./s or more in the range of 500˜950° C. The size (gauge portion) of the tensile sample obtained from the hot upsetting product was diameter 4.1 mm×length 16.3 mm.

FIGS. 1A and 1B (which is an enlarged view of FIG. 1A) show microstructures of non-QT steel after hot forging at 1150° C., and FIGS. 1C and 1D (which is an enlarged view of FIG. 1C) show microstructures of non-QT steel after hot forging at 1050° C. FIGS. 1A to 1D show the results of decreasing the hot deformation temperature from 1150° C. to 1050° C. under conditions of the rapid cooling (isothermal holding) temperature being set to 600° C. When the hot deformation temperature was decreased in this way, the size of pearlite nodules was reduced and, thus, the ferrite fraction was increased. As observed in the enlarged views of FIGS. 1B and 1D, however, the pearlite colonies and the lamellar structure became coarse instead. Without being bound by theory, this is believed to occur because the degree of supercooling for pearlite transformation is decreased at the decreased hot forging temperature, thus reducing the driving force for pearlite isothermal transformation. The driving force for pearlite transformation is increased in proportion to an increase in the hot forging temperature. However, if the hot forging temperature is excessively high, the probability of generating the proeutectoid ferrite or continuous cooling pearlite transformation during rapid cooling may increase. Furthermore, it is undesirably easy to make the hot forged structure coarse. Accordingly, in embodiments of the present invention, the preferred hot forging temperature is set to 1150° C., which falls in the range of 1000-1200° C.

FIG. 2A shows the microstructure obtained by cooling the hot forged steel to 600° C. at 2° C./s and then air-cooling it, and FIG. 2B shows the microstructure obtained by cooling the hot forged steel to 600° C. at 10° C./s and then air-cooling it. Test samples of high-temperature compression deformed using Gleeble were cooled to 600° C. at 2° C./s and 10° C./s in two different single-cooling processes, and then air-cooled to room temperature to provide the microstructures which were observed using SEM.

In the case where the cooling rate is relatively slow (2° C./s), a typical pearlite-ferrite structure and a comparatively coarse pearlite nodule structure (pearlite enclosed with grain boundary ferrite) are seen. On the other hand, in the case where the cooling rate is fast (10° C./s), relatively fine pearlite nodules are observed. However, the proeuctectoid ferrite formed along austenite grain boundaries has a low fraction and is discontinuously formed, and thus the pearlite nodules are not obviously distinguished. Furthermore, as shown, the pearlite structure formed thereafter is a very irregular lamellar structure. This is because both proeutectoid ferrite and pearlite are incompletely formed at a fast cooling rate.

The above results demonstrate that the ferrite transformation is suppressed in proportion to an increase in the cooling rate, and the pearlite transformation also takes place, which provides results according to a conventional technique (a single controlled cooling process). As further demonstrated, when the cooling rate is fast (e.g., a level of about 10° C./s or more, the formation of proeutectoid ferrite during cooling may be considerably suppressed, and thus the austenite hot deformation structure may be supercooled to the low-temperature pearlite isothermal transformation range. The reason that the high-temperature ferrite transformation of the inventive steel may be suppressed at a fast cooling rate of 10° C./s or more is that a considerable amount (1.2 wt %) of Mn is contained in the steel of the present invention. Mn is known to shift the austenite-ferrite CCT diagram rightwards.

FIG. 3A shows the microstructure obtained by cooling the hot forged steel to 600° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it, FIGS. 3B and 3C show the microstructures obtained by cooling the hot forged steel to 550° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it, and FIG. 3D shows the microstructure obtained by cooling the hot forged steel to 500° C. at 10° C./s, isothermally holding it for 10 minutes and then air-cooling it. The microstructures of the test samples obtained after compression deformation using Gleeble, and then multi-stage rapid cooling in accordance with embodiments of the present invention were observed.

In the first stage of multi-stage rapid cooling according to the present invention, the austenite structure which was high-temperature compression deformed is rapidly cooled to 600° C. or less at a fast cooling rate of 10° C./s. The supercooled austenite deformation structure in the pearlite transformation range was, thus, obtained. In accordance with the present invention, this rapid cooling is performed to maximally suppress formation of proeutectoid ferrite during continuous cooling, and to further maximally prevent the continuous cooling transformation of pearlite. The second stage is performed to induce the pearlite isothermal transformation at low temperature by isothermally holding the supercooled austenite deformation structure at 600˜500° C. for a short period of time (5˜30 minutes). Upon pearlite isothermal transformation, unlike continuous transformation, the degree of supercooling is very high and the rate of nucleation of the colonies is greatly increased, thus obtaining an ultrafine grained pearlite colony structure. In addition, the lamellar spacings become fine. In the third stage, the ultrafine grained pearlite colony structure is air-cooled. In this stage, V-carbonitride is deposited on ferrite in pearlite.

The steel obtained after treatment at a rapid cooling temperature, namely an isothermal holding temperature of 600° C. (for 10 minutes) shows a typical pearlite-ferrite structure, unlike steel air-cooled without isothermal holding. In this case, a considerable amount of proeutectoid ferrite is formed around austenite crystal grains. Accordingly, the pearlite structure formed after that is a very fine and regular lamellar structure. However, the size of pearlite nodules formed at this time is larger than when air-cooled at 600° C.

Without being bound by theory, it is believed that this occurs because the pearlite nodules become coarse during isothermal holding. In order to further suppress the formation of proeuctectoid ferrite, the rapid cooling temperature, namely the isothermal holding temperature, was further decreased to 550° C. (for 10 minutes). As shown in the drawings, the formation of proeutectoid ferrite was extremely suppressed (less than 5%), thus forming the ultrafine grained pearlite (pseudo-pearlite) structure. As such, the pearlite structure may be observed to have very fine lamellar spacings and an ultrafine grained colony structure (in which the lamellar directions are different). When the rapid cooling temperature, namely, the isothermal holding temperature was further decreased to 500° C., the pearlite structure disappeared, and a structure close to bainite was formed.

As is apparent from the above results, in the case where the cooling rate is 10° C./s, when the rapid cooling temperature is decreased to 550° C., the formation of proeutectoid ferrite can be maximally suppressed for the first time. In addition, when the pearlite isothermal transformation occurs at a low temperature of 550° C., the pearlite colonies and the lamellar structure become remarkably fine. This is because the formation of continuous cooling pearlite is maximally suppressed for the first time at a rapid cooling temperature of 550° C., and the very large degree of supercooling for pearlite isothermal transformation may be obtained.

When the rapid cooling temperature was further decreased to 500° C., the pearlite structure was not formed, but instead a structure close to bainite was formed. Thus, the pearlite isothermal transformation temperature is preferably set to about 500˜600° C. As such, the isothermal holding time is preferably in the range of about 5˜30 minutes. If the time is shorter than about 5 minutes, pearlite isothermal transformation does not sufficiently occur. In contrast, if the time is longer than about 30 minutes, the pearlite colony structure becomes excessively coarse.

FIG. 4 is a graph showing the effects of isothermal holding temperature on Vickers micro-hardness. The hardness of steel according to the present invention, which was rapidly cooled to 600° C. at a rate of 10° C./s, isothermally held at 600° C. for 10 minutes and then air-cooled, was similar to that of comparative steel obtained after rapid cooling to 600° C. and then air-cooling. However, the hardness of the steel, which was rapidly cooled to 550° C. and isothermally held at 550° C. for 10 minutes according to the present invention, was remarkably increased.

Further, in the case where the rapid cooling temperature was decreased to 500° C., hardness was considerably decreased. This hardness distribution coincides with the microstructure distribution as shown in FIGS. 3A to 3D. The reason why the peak of hardness is shown at a rapid cooling temperature of 550° C. is that the transformation of proeutectoid ferrite and continuous cooling pearlite is maximally suppressed at this temperature, thus forming the ultrafine grained pearlite colony structure (FIGS. 3A to 3D). Such an ultrafine grained pearlite colony structure is presupposed to have superior toughness, as well as high strength. In order to confirm this presupposition, a hot upsetting test was performed using a hot press.

FIGS. 5A and 5B show the microstructure of steel hot forged at 1100° C. and air-cooled, and the microstructure of steel hot forged at 1100° C. and subjected to multi-stage rapid cooling. FIGS. 6A and 6B show the other comparison results of FIGS. 5A and 5B. FIGS. 5A and 5B illustrate, using an optical microscope, the results of observing the structure of the steel hot upset to a deformation of 1.2 at 1150° C. and then air-cooled, and the structure of the steel (which is non-QT steel according to the present invention) hot upset under the same deformation conditions and then subjected to multi-stage rapid cooling according to the present invention in which the pearlite isothermal transformation temperature is 550° C.

The comparative steel shows a typical pearlite-ferrite structure. The pearlite nodules enclosed with ferrite formed along crystal grain boundaries are very visible. The pearlite nodules are composed of bundles of colonies like fine crystal grains. On the other hand, the non-QT steel according to the present invention manifests a pseudo-pearlite structure composed mainly of pearlite with maximally suppressed formation of proeutectoid ferrite. Furthermore, the pearlite structure of the non-QT steel according to the present invention demonstrates only the ultrafine grained colony structure without forming the pearlite nodules.

FIGS. 6A and 6B show the microstructures of the comparative steel and the inventive steel, as observed using SEM. In the inventive steel, the size of colonies was drastically decreased, and the lamellar spacings became fine.

As demonstrated, by applying the multi-stage rapid cooling process according to the present invention, the ultrafine grained pearlite (pseudo-pearlite) forged steel is manufactured, in which the proeutectoid ferrite fraction is extremely suppressed to 5% or less and the size of colonies is 5˜10 μm and without forming the pearlite nodules.

The following table shows results of conducting a tensile test on the comparative steel obtained after hot-upsetting and air-cooling and the inventive steel obtained after upsetting, rapid cooling to 550° C. and isothermal holding. In the case of the ultrafine grained pearlite (pseudo-pearlite) forged steel according to the present invention, strength was drastically increased (yield strength (YS) increased by 13%; tensile strength (TS) increased by 8%) and the elongation was equal to or greater than that of pearlite-ferrite forged steel as the comparative steel. According to the present invention, the high-strength high-toughness V-MA pearlite medium-carbon forged steel having an elongation of 18% and a tensile strength of 970 MPa can be manufactured. This is possible because the ferrite phase was excluded from the conventional pearlite-ferrite forged steel, and the continuous cooling pearlite transformation was suppressed due to the application of the multi-stage rapid cooling according to the present invention, thereby forming the ultrafine grained pearlite colony structure.

TABLE 2 Conventional Inventive YS (MPa) 623 704 TS (MPa) 901 971 Elongation (%) 17% 18%

As described hereinbefore, the present invention provides non-QT steel having an ultrafine grained pearlite structure and a method of manufacturing the same. According to the present invention, a new concept of ultrafine grained pearlite (pseudo-pearlite) non-QT steel can be ensured, in which limitations of pearlite-ferrite forged steel are overcome and strength and toughness are remarkably enhanced.

In the non-QT steel having an ultrafine grained pearlite structure according to the present invention, ferrite transformation is suppressed, continuous cooling pearlite transformation is maximally suppressed, and low-temperature pearlite isothermal transformation is induced, thereby maximizing the driving force for pearlite transformation. Consequently, the rate of nucleation of pearlite colonies is greatly increased, thus obtaining ultrafine grained colonies and lamellar structures.

The ultrafine grained pearlite (pseudo-pearlite) forged steel has an elongation which is equal to or higher than that of conventional pearlite-ferrite forged steel (for example, an elongation of about 18%), and also has yield strength increased by 13% or even greater and tensile strength increased by 8% or even greater, resulting in non-QT steel having a high strength (e.g. about 970 MPa) and high toughness.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

1. A method of manufacturing non-quenched and tempered steel having an ultrafine grained pearlite structure, the method comprising: hot forging a steel material so as to be high-temperature compression deformed, thus obtaining a hot forged body; rapidly cooling the hot forged body to a low-temperature pearlite transformation range, thus obtaining a supercooled hot forged body; isothermally holding the supercooled hot forged body in the low-temperature pearlite transformation range, thus isothermally transforming it; and air-cooling the hot forged body.
 2. The method of claim 1, wherein the hot forging is performed at about 1000˜1250° C.
 3. The method of claim 1, wherein the rapidly cooling is performed by cooling the hot forged body to about 500˜600° C. at a rate of about 10° C./s or more.
 4. The method of claim 1, wherein the isothermally transforming is performed by isothermally holding the supercooled hot forged body at about 500˜600° C. for about 5˜30 minutes.
 5. The method of claim 1, wherein the rapidly cooling is performed by cooling the hot forged body to about 500˜600° C. at a rate of about 10° C./s or more, and the isothermally transforming is performed by isothermally holding the supercooled hot forged body at about 500˜600° C. for about 5˜30 minutes.
 6. The method of claim 1, wherein the step of air-cooling the hot forged body provides a non-quenched and tempered steel having an ultrafine grained pearlite structure composed mainly of Fe and, further, about 0.43˜0.47 wt % of C, about 0.15˜0.35 wt % of Si, about 1.1˜1.3 wt % of Mn, greater than 0 wt % and up to about 0.03 wt % of P, greater than 0 wt % and up to about 0.04 wt % of S, greater than 0 wt % and up to about 0.3 wt % of Cu, greater than 0 wt % and up to about 0.2 wt % of Ni, about 0.1˜0.2 wt % of Cr, greater than 0 wt % and up to about 0.05 wt % of Mo, about 0.08˜0.15 wt % of V, greater than 0 wt % and up to about 0.02 wt % of Al.
 7. A non-quenched and tempered steel having an ultrafine grained pearlite structure, produced by subjecting a hot forged steel material at about 1000˜1250° C. to rapid cooling to a pearlite transformation range of about 500˜600° C., isothermal holding at the corresponding temperature for about 5˜30 minutes, and then air-cooling.
 8. The non-quenched and tempered steel of claim 7, wherein the hot forged steel material is rapidly cooled to the pearlite transformation range of about 500˜600° C. at a rate of about 10° C./s or more.
 9. The non-quenched and tempered steel of claim 7, wherein the non-quenched and tempered steel formed after air-cooling has a proeutectoid ferrite fraction of about 5 vol % or less and pearlite colonies having a size of about 5˜10 μm. 